1. Field of the Invention
The present invention relates to a semiconductor light emitting device used as a semiconductor laser device, as a blue light emitter which is one of the elements of a display panel for use in displays of various electronic apparatuses, as a blue light emitting device (LED) used individually in a display apparatus, as a signal reading and writing light emitting device for use in a compact disk (CD) player and a laser disk (LD) player, and as a light emitting device for use in a bar code reader.
2. Description of the Prior Art
FIG. 1 schematically shows the basic structure of such a semiconductor light emitting device and the condition of a corresponding energy band. The general structure of the semiconductor light emitting device is as follows: a semiconductor film B of a P-N junction structure is formed on the surface of an N-type semiconductor substrate by MBE (molecular beam epitaxy)-growing an N-type semiconductor layer B.sub.1, an active layer B.sub.2 and a P-type semiconductor layer B.sub.a in this order, and metal electrodes E.sub.1 and E.sub.2 are formed on the N-type substrate A and on the P-type semiconductor layer B.sub.3 which is the top layer of the semiconductor film.
As well known, the energy band of the semiconductor device of the above-described structure is of a configuration such that the energy levels of the N-type semiconductor film B.sub.1 and the P-type semiconductor layer B.sub.3 are high and an energy level trough is formed at the active layer B.sub.2 which is a P-N junction. Consequently, when a bias voltage is applied between the electrodes E.sub.2 and E.sub.1 in a forward direction, the carriers, i.e. holes h and electrons e injected by the voltage application recombine by being shut up in the active layer B.sub.2 where the energy level is low, so that natural light is emitted therefrom. In a semiconductor laser, when the exciting current exceeds a threshold value, light resonates between the parallel end surfaces of the active layer B.sub.2 to cause a laser oscillation.
Conventionally, in manufacturing a semiconductor light emitting device which emits blue light, the semiconductor film B is formed by epitaxially growing a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe directly on a GaAs substrate A, or through a group III-V film grown on the GaAs substrate A and having the same conductivity as that of the GaAs substrate A.
FIG. 2 shows a conventional semiconductor light emitting device. The conventional device shown in the figure is what is called a blue light emitting semiconductor laser of ZnSe in which a group II-VI semiconductor film 42 of ZnCdSSe or MgZnCdSSe is formed on an N-type GaAs substrate 41.
The group II-VI semiconductor film 42 is formed by MBE-growing an N-type ZnSe layer 43 which is a buffer layer, an N-type ZnSSe layer 44 which is a clad layer, a ZnCdSe layer 45 which is an active layer 45, a P-type ZnSSe layer 46 which is a clad layer and a P-type ZnSe layer 47 which is a buffer layer in this order on the substrate 41. On the P-type ZnSe layer 47 which is the top layer of the group II-VI semiconductor film, a metal such as Au is directly deposited to form an electrode 48. Reference numeral 49 represents an electrode formed on the reverse surface of the substrate 41.
In the semiconductor device of the above-described conventional structure, as is obvious from the energy band configuration shown in FIG. 3, there is a considerably large energy band gap between the N-type GaAs substrate 41 made of a group III-V semiconductor and the N-type ZnSe layer 43 of a group II-VI semiconductor film 42. As a result, a barrier potential .DELTA.V is generated. The broken arrows in FIGS. 1 and 3 show the flows of electrons e.
Since a considerably high voltage must be applied between the electrodes 48 and 49 to obtain a current which causes the electrons e to go over the barrier potential .DELTA.V, not only the power required to drive the device increases but also amperes of currents flow in the device. Since the current density in the device is very high, it is inevitable that the device generates heat while being driven.
Such problems can be similarly caused when a P-type GaAs substrate is used. In this case, it is necessary to apply between the electrodes a voltage required to obtain a current which causes holes h to go over the potential barrier between the P-type GaAs substrate and the II-VI semiconductor film. The problems of the power consumption and the heat generation when the device is driven are caused like in the case where the N-type GaAs substrate is used.
Moreover, in a structure where a group III-V semiconductor film of the same conductivity as that of the GaAs substrate is grown on the GaAs substrate, since there is a band gap between the group III-V semiconductor film and the group II-VI semiconductor film, the same problems are caused.